US2348184A - Electron flow device - Google Patents

Description

Z. J. ATLEE I'AL ELECTRONy FLOW DEVICE Filed Dec. ll'. 1941 INVENTORS: Zed [H May' 9, 1944.

Patented May 9, 1944 ELEc'rRoN FLOW DEVICE Y Zed J. Atlee, Elmhurst, Ill., and Robert French; o. ,l i.

Wilson, Washington, D. C., assgnors to General Electric X-Ray Corporation,

poration of New York V Application December *11, 1941, Serial No.422,530 Y 9 claims. (c1. 25o-142') The present invention relates in general to electronics and has more particular reference to an improved X-ray generator.

An important object of the invention is to provide an electron ow device embodying an electron emission source adapted to produce or emit electrons in an extremely eiiicient fashion, thereby improving the operating characteristics of the electron iiow device.

Another important object is to provide an electron emission source of unusually high eiiiciency; a further object being more particularly to provide an efficient electron emission source comprising an element adapted to be heated for electron emission at relatively low temperature; a further object being to utilize an emission element comprising thorium for highly eflicient electron emission.

Another important object is to provide an electron emission element comprising thorium supported on a suitable mounting, such as a tungsten lament; a further object being to support the j tron flow device embodying the improved elec-` tron emission element as a cathode, in combination with a co-operating anode, enclosed in a sealed evacuated envelope, and to preserve the thorium by providing means for maintaining the exhausted envelope at a high degree of vacuum of the order of 0.02 micron of mercury or better; a further object, to this end, being to provide gettering means within the envelope and preferably comprising a part of the cathode structure, said gettering means being operable at intervals during the service life of the device to maintain the desired degree of vacuum within the envelope for the preservationof the electron emission element for operation at high efficiency.

Another important object is to utilize a suitable evaporable metallic substance, such as barium, for gettering purposes and to provide means for supporting the gettering substance Within the envelope and for heating the substance for the evaporation thereof; a further object being to support the gettering substance on a mounting comprising a wire within the envelope and hav- Chicago, Ill., a coring connections* extending outwardly of the envelope whereby to heat the Wire by energizing it from an external source of electrical power for the selective and controlled Vevaporation of the getteringsubstance.I

Another important' object is to provide means Within the envelopaoperable as an ionization gauge,.for the determinationof vacuum conditions within the envelope during the-operating life ofthe device; a further object being to form the gauge as a platehaving a connection extending outwardly of the sealed envelope for attachment in an .external measuring circuit and operable as an ionic collector, said plate serving, in conjunction with the external measuring circuit, to measure the degree of ionization and hence vacuum conditions within the envelope; a further object being to form the ionization gauge plate as a part of the cathode structure of the device; a still further object being to utilize the said gauge plate as a housing or'enclosure for the gettering means to prevent end products of the gettering operation from settlingV upon the anode and cathode elements of thedevice.

Another important object isto provide an' lX- ray generator comprising an anode and a cooperating cathode enclosed within a sealed evacuated envelope, wherein the cathode comprises a thoriated and preferably carboniz'ed tungsten lament; another object being to provide Vineansforming an ionization gauge within the envelope for the determination of vacuum conditions therein; a further object being to provide getteringmeans operable Withinthe envelope to maintain a'suiiicient degree of vacuum within the envelope for the preservation of the thoriated filament in condition for satisfactory operation throughout an extended generator service life; a still further object beingv to utilize an evaporable gettering substance, in combination with means forcontrolling the evaporation thereof for gettering V purposes Within the envelope and to form the ionization gauge as a plate enclosing the gettering substance in order to prevent the same, when vaporized, from depositing'upon the anode and cathode of the generator.

These and, numerous otherimportant 'objects, Y'

' outwardly ofthe `envelope.V

of the invention are not necessarily limited to generators but may be utilized Yto advantage gen.-

erally in electronic devices embodyingmeansV forming an electron emitting. source.

The generator II, as shown on thedrawings,

Vmay comprise a-sealed envelope I3 'con'tainirig an anode I5 and a co-operating cathode I'I. The

Venvelope I3 may be formed ofA any suitable eri-*-v velope material and may be arranged in any stantially lower than that of tungsten. Conse-i quently, a thoriated lament of the character herein described is characterized by the ability to emit an effective stream of electrons at a temperature substantially lower than the eifective operating temperature of a-pure tungsten lament. As a consequence, an electron flow device embodying a thoriated filament, in accordance with the teachings of the present invention, is .capable of functioning at a lower temperature and hence at appreciably higher efciencythan is the case with devices containing electron emission elements of the character hereconvenient or preferred form. As shown, theV .envelope mayL comprise a Y, tubular glass shell having releitrant ricksfl at'xthe oppositeends of fthe shell,v the Aari'odeIi'and cathode I1 being, respectively, supported on' sfaid'necksY atv the pposit'e ends of 'the envelope. The eoksfIS de-y fine yopenings which `are closedV by Vseal members `2 and f23 o`f`g`enerll'y vcllp-shaped cchguratn. These membersarefpreferably 'formed of conductinginaterial ariel each has 'an annular" rim Whil vIIS a. IIS'SFISOfInI ,Seal '25' With the inwardly facing fend of th'ec'k ,I9 on which it is mounted Vto thus sealA tlie 4end of the envelope. The nii-interfiere irrdfof theenveiope, carries the anode I'fwi iclras'shown, may corn'- prise a'rotaiy'ah'ide" vehtpro'vidiriga target I ings, turnably supported "which projects outti/army` 'of the l vve oeitliiolgh aifopeirig formed inthe Vsfealme'mber `2 3, 'thestem' 231 being ysealed in saidA openin'gl: i The Vanode 'may be rotated within" the envelope by electric 'motor means iricludingi'the stationary'stator 26"niunted on and The InembferZL atjthe :her endf the en'- velope, carriesthe1 c'athcde'll Which, as shown, preferalc/1yy comprisesalplatlikehelad '28 formed with apocket in'ivvhich'ifsjmounte'd an electron emissionfilanjien1;' 32` imposition to emit electrons inthe directiolri'"of anode, r`whener'iergized. y'lhejilan'ilentic'mp sf a highly""eflcient"electron Vemissio"n1sur'cfeY whenA 'electrically cimiteri', yand tghthisrfjerid, it` comprisesga suitablefsupport element` for thoriuijri.u Preferably, y the" thorium lsupport comprisesfa iilamentary wire of tungsten carrying Vthe thoriumf-atthe surfaces of the support larietsj s Y Thegelectronmeini' lion Vfilament 32 Y'may be formed by mixingthorium" in quantities of the order of 2% with tungsten, 'sintering the mixture, and then 'drawingfit` through a suitable die to produce a fllament'aryrrwire comprising tungsten with the't'horium component substantially uniformly distributed in theV filament. By operating the lair'ient at high temperature, the thorium is drivento thefsurfacer ofthe filament -to form a thoriumviilmfon the surfacespcfthe ilament. The heating'of the filament to'v form a thorium lmthereon neednot be accomplished untii'nienianiet -hasfbeen'assmbled in the eiectronA flow device r'and'rnayfber accomplished as a VpartA of une process of conditioning the nnished device for service, The` process ofA thusV conditioning the lamentjmrelycomprises the heating offthe filamenty toa; temperature, of the order O'f' the" mltng'ierpe'rature V0fhlngsten-` Thorium functions? forelectron emission when tofore' known;

In `order to aid' in holding the thorium on the filament during the service life of the device and to thereby maintain the cathode lament as a highly eicient electron source, it is desirable t0 carbonize the filament in order to provide a surface' 'coatingiof carbide, specifically aitungsten carbide layer where thev lam'ent comprises tungsten, sincel thorium' has a greater affinity andwill begheldjmore securely `on the filament by theycarbide than'by pure' tungsten".V To this end, theA iil'amentvis preferably carbo'nized,v by heating it'toa temperature ofthe order of '2,000 degrees centigrade in a hydrogen bottleor container in the presence of a carbo'nizinggals such as acetylene.' vIn thisfashion, asheath of tungsten 'carbide maybe applied., tothe surfaces of the filament to` serve as afoundation forV holding the thorium on'tlie'filament. l The carbonizing process should be limitedto the surfaces and should not penetrate completely to the center of the lament in orderv to4 avoid extreme brittleness. In performing thecarbonizingfprocess, the degree of carbonization may be ldetermined by measuring the electrical resistance of the filament frointime totim'e, the resistance of tungsten carbide'being substantiallyhigher thanthat i of puretungsten, so'that a desired? degree of carglobules of lglass which notV only support the conductors butalso insulate the same from the grommet and hermetically 'seal the openings around the conductors.1 -The cathode'head 28 Carries a tubular skirt'l secured at'orel Aeri'd on the headl and extending at the other eicl in position enclosing? theIgless-tolmetal seal i25. The Skirt is alsosecu'red t0` the Sealhmeans 2| as'by screws 31 and thus enclos'esy a space'g between the seal 'member `2| andtheheadZB, which' spacey isfin' opencx'rinunication with the interior ofVthe-envlope "through the annular spacedened betweenthe end of the sli-irt 41 and the Yglass-tolr'ne'tal seal 25-.- vWithin this space, means is provided rforperforming a gettering operation in ordertor insure the main-te'- nance of a fhigh degree of vvacuum Within the envelope. y Y

electrons emitted at the cathode and traveling to and impinging upon thef anode. When -the fiow device comprises an X-ray generator, the stream of electrons pass from the cathode and impinge on the anode target, thereby generating X-rays at the anode.

For eicient operation of the device as an X-ray generator, the atmosphere within the envelope I3 is preferably maintained at pressures of the order of 0.02 micron lof mercury, and the maintenance'of low vacuum conditions is of particular importance` where the device embodies a thoriated filament in accordance Vwith the present invention. The operation of an electron flow device results `in, the ionization of any gas particles within the envelope with which an electron may collide during its travel between cathode and anode. Such a collision results in the production of a positively charged ion which may impinge upon the cathode filament and dislodge therefrom a particle of thorium. In order, therefore, to preserve the thorium in situ during an extended service life, it is necessary to reduce the possibility of ionic impact on the cathode filament and consequent dislodgment of thorium by eliminating, as far as possible, the presence of gaseous matter within the envelope. To this end, means is provided for insuring high vacuum conditions within the envelope throughout the service life of the device. To this end, the envelope is first thoroughly evacuated as a part of the manufacture of the device, evacuation being accomplished after the parts of the device have been assembled and sealed in the envelope. During the evacuation of the envelope, which is preferably accomplished by attaching the same to suitable exhaustmeans, such as an exhaust pump, the device and all of its operating parts are strongly heated in order to remove all impurities, including such gases as may be occluded in the anode, cathode and envelope Walls. After the device has thus beenexhausted to the greatest possible extent. theenvelope maybe sealed off and, as a final operation to remove all remaining impurities,

Va gettering operation may be performed within the envelope. According to the present invention, gettering is accomplished by vaporizing a suitable gettering material within the envelope, more particularly within the space 49.V Barium may be utilized as av gettering material, and by vaporizing the same within the envelope, all remaining impurities may be caused to unite with the barium vapor to form innocuous solid gettering end products which, upon condensation of the gettering material, will become deposited within the envelope I3. By arrangingthe head 28 and the skirt 41 as a housing enclosing the chamber 49 and performing the gettering operation therein, gettering end products w-ill be deposited upon the internalsurfaces of the housing defining the gettering chamber 49 and will thus be prevented from becoming deposited either upon the wall surfaces of the envelope I3 or upon the anode and cathode.

. Barium or other suitable gettering material maybe supportedwithin the chamber 49 and evaporated therein by any suitable or preferred means. Preferably, however, the gettering material is supported by a carrier wire 5I electrically connected at one end on the seal means 2| and at the other end on a conductor 53, the carrier wire 5I having portions intermediate the anchored ends thereof arranged in loops within the chamber 49. The conductor -53 extends through an opening in the seal means 2| and is 75 sealed, as by means of a globule of glass, in'a grommet 59 similar to the grommets 29V and, like them, sealed in the member 2|. t

The gettering material may be supported in any suitable or preferred fashion upon the carrier wire 5I, which preferably is a hollow conductor wire enclosing the gettering materialand havingweakenedwall portions. By passing electrical current through the wireY 5I, as byV con- -necting a suitable electrical power source. 51 between a stud 45, mounted on and electrically connected to theseal 'means'ZL and the .con- .ductor 53 outwardly ofthe envelope, the hollow carrier wire 5I may be heated Vwithin the envelopeto thereby vaporize the gettering material carried by the wire. The material thus vapor- Vized will be projected outwardly ofthe wire through its weakened wall portions to perform the gettering operation within the`chamber'49, resulting end products being deposited upon the rner walls of the tubular skirt 41 and the head The gettering circuit between the stud 45 and the conductor 53, in addition to the power source 51, 'preferably includes a meter 6I for measuring current flow in the gettering circuit, an adjustable control rheostat 63, Land a control switch for regulating current flow in the gettering circuit for gettering control through regulation of vaporization of the' gettering material inthe chamber 49.H It will be apparent, of course, that sufficient gettering material may be supplied l within the chamber 49 to enable successive'geb tering operations to be performed within the envelope at intervals during the'service life of the device to thereby periodically re-establish desired vacuum conditions within theV envelope in the event that the vacuum be impaired-during the operating life of the device.

The device of the present invention also includes means for testing and measuring vacuum conditions within the envelope from time to time by'ionization gauge means, including the head 28 and skirt 41 which, it will be noted, are electrically connected on the stud 45 and otherwise insulated from the cathode filament 32 and the anode I5.

The degree of vacuum within the envelope I3 is a function of the number of gaseous particles present within the envelope, and the number of such'particles, in turn, is a function of the degree; of ionization developed as a result' of collision between electrons emitted by the cathode lament with any such gas particles present within the envelope. By measuring the degree of ionization within the envelope resulting from a measurable electron flow between cathode filament and anode, itis possible accurately to determine the presence of gas particles Within the envelope and thereby determine the vacuum condition at any time during the service life of the device. This may be accomplished by promoting a known electron flow between cathode filament and anode and then measuring the resulting ionization by collecting resulting positive ions ony the head 28 and skirt 41 and quantitatively measuring the ions thus collected. To this end, an ionization gauge circuit comprising a meter 61 and a power source 69 may be connected between either of the conductors 35 and the anode stem 24, preferably. under the'control of a switch 1I and an. adjustable current flow regulating device, such as a rheostat 13. By adjusting the rheostat, a

known electron now, as indicated by the mete;`

B'Lmay be established between the cathode and anode; n

By applying a negative bias to the head 2.8 and skirt 41 with respect to the cathode lament -32,.as by means of a power source connected betweenv one ofthe cathode leads 36 and the stud.45,.which is electrically connected with the head 28"and the skirt 41, positive ions, produced Within the envelope by electronic collisionkwith gas particles, will be caused to impinge upon the head 28l and skirt 41,.the extent of such ionic impingement being accurately measurable upon a meter 11 included with the power source 15 in the biasingcrcuit. between the stud 45. and the cathode lament 32. Whenever the meter Tl shows' ionization aboveV a predetermined minimum,V itA will be known that the desired vacuum condition within the envelope I3 has been impairedand that the .device should be reconditionedxby the performance of a gettering operation in order to recondition the device for operation under ideal conditions for the preservation of thev thoriated cathode lament.

It will be understood, of coursethat the cir'- cuits shown in dotted lines, namely, the ioniza. tion-circuit including the meter S'Ltheionization measuring circuit including the meter 11, and the gettering circuitlincluding the meter (il,V are connected with the device in the manner shown only when it is desired to measure vacuum conditions within Vthe envelope I3 andto reconditionrthe device by the performancev of a gettering operation. When the device is in service as an X-ray generator, the cathode leads 36 will be connected by conductors toa suitable source of energizing power 8l and the anodeand cathode will be connected with a source of unidirectional power 83.

It is thought that the invention and its numerous attendant advantages will `be fully understood from the foregoing description, and itis obvious thatnumerous changes may be made in the form, construction and arrangement ofthe several parts without ldeparting from the' spirit or scope ofthe invention, or sacrificing any of its attendant advantages, the formA herein disclosed being a preferred embodiment for the purpose of illustrating the invention.

The invention'is hereby claimed as follows:

l. An X-ray Vgenerator comprising lan evacuated envelope at a pressure of the order of 0.02 micron of mercury or lower, and 4an Velectron emission element comprising thorium and. a sheath of tungsten carbide thereon in--saidenvelop'e and subject to the low pressure conditionsjprevailing therein. u v u 2. An electron flow device comprising an evacuated envelopeV at a pressure of the order of 0.02`fmicronof mercury or lower, andan electron emission element comprising thorium amd` .a sheath 4of tungsten carbide'thereon in 'said envelope and subject to the lowA pressure conditions prevailing therein. Y

3. An X-ray generator comprising ani-evacuated envelope at a pressure of the order of 0.02 microno mercury or lower, and a cathode-comprising a support formed with a pocket, andY an electron emission element in sa-id pocket comprising thorium and a sheath of tungsten carbide thereon and subject to the low pressure condi.- tion'sprevailing in Vsaid-envelope, saidv cathode and-said element being arranged and conformed to-direct the ow of electrons from saidcathode.

"LAnA X-r'ay generator comprising an evacuatedi'envelope `at a pressure of the order of 0.02 micron of mercury or lower, and a cathode comprising a support formed with a pocket, and an electron emission element in said pocket comprising thorium and a sheath of tungsten carbide thereon and subject to the low pressure conditi'oris prevailing in said enevelope, said cathode and said element beingl adapted to aid in focusing the .impin'gement of electronsvdischarged from said cathode upon a target within said envelope.

5. An X-ray generator comprising an evacuated envelope at apressure of the order of 0.02 micron of mercury or lower and acathode comprising a support formed with a pocket and an electron emission element in said pocket, said element comprising carbonized tungsten and thorium diffusable in the carbonized tungsten and being subject to the low pressure conditions prevailing in said envelope, said' cathode and said element being adapted to aid in focusing the impingement of electrons from said cathode upon a target within said envelope.

6.; An X-ray generator comprising an evacuated envelope at a pressure of the order of 0.02 micron of mercury or lower and a cathode comprising a support formed with a pocket and an electron emission element in said pocket, said element comprising carbonized tungsten having a thorium film thereon and being subject to the rlow pressure conditions prevailing in said envelope, said cathode and said element being adapted to aid in focusing the impingement of electrons discharged from said cathode upon a target within said envelope.

7'. An- X-ray generator comprising an evacuated envelope at a pressure of the order of 0.02 micron of mercury or lower and a cathode cornprising a support formed with a pocket and an electronv emission element in said pocket, said element comprising thoriated tungsten carbide and being subject to the low pressure conditions prevailing in said envelope, said cathode and said element being adapted to aid in focusing the impingement of electrons discharged from said cathode upon a target within said' envelope.

8'. An X-rayy generator comprising an evacuated envelope at a pressure of the order of 0.02 micron` of mercury or lower, and a cathode comprising a support formed with a pocket, and an electron emission element in said pocket comprising thorium andA subject to the low pressure conditions prevailin-g in said envelope, in which the electron ow through the generator is controlled by the temperature of said element, said cathode and said element being adapted to aid in focusing the impingement of electrons discharged from said cathode upon a target Within-said envelope.

9. An X-ray generator comprising an evacuated envelope at a pressure of the order of 0.02 micron of mercury or lower, an anode, and a cathode having apocket, and an electron emission element comprising thorium mounted in said pocket and subject to the low pressure conditions, prevailing in said envelope, said cathode being adapted to aid in focusing the impingement of electrons upon said anode.

ZED J. ATLEE. ROBERT FRENCH WILSON.

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